1. Field of the Invention
The present invention relates to a motor driving apparatus for driving a motor.
2. Related Art
JP-A-4-101694 discloses a conventional motor driving apparatus which performs switching control of a power transistor connected to three-phase coils of a motor to control a current that flows in the three-phase coils. The conventional motor driving apparatus is described by means of FIGS. 10 and 11.
In the conventional motor driving apparatus shown in FIG. 10, a rotational position detector 11 detects a position of a rotor of a motor 7 by use of hall elements 11U, 11V, 11W to generate rotational position detection signals H1, H2, H3. A hall amplifier 11b amplifies the rotational position detection signals H1, H2, H3 and shapes waveforms to generate and output sinusoidal signals S1, S2, S3 to a multiplication circuit 2a. 
A current detection circuit 4 has a detection resistor for detecting a drive current Im that flows in the motor 7 as a voltage value, and the current detection circuit 4 outputs a current detection signal Vcs. An error amplifier circuit 6a amplifies the difference between the current detection signal Vcs and a drive command signal Vec that controls the drive current Im inputted into an input terminal 5, and converts the amplified difference into a current to output the current as an error amplifier current signal Ir to the multiplication circuit 2a. 
The multiplication circuit 2a multiplies the sinusoidal signals S1, S2, S3 outputted from the hall amplifier 11b by the error amplifier current signal Ir outputted from the error amplifier circuit 6a to generate and output input command signals Vuin, Vvin, Vwin.
The drive circuit 3 has a switching control circuit 32 and a power supply circuit 33. The switching control circuit 32 compares the input command signals Vuin, Vvin, Vwin with a carrier wave Vtri which is a triangle wave inputted into an input terminal 31, and performs pulse width modulation to output switching control signals UPWM, VPWM, WPWM. The power supply circuit 33 performs switching control of power transistors 331 to 336 based on the switching control signals UPWM, VPWM, WPWM in a gate drive circuit 337, and generates drive voltages Uout, Vout, Wout from a power supply 338 to output the generated voltages to three-phase coils 71, 72, 73 of the motor 7.
The conventional motor driving apparatus with the configuration as thus described controls amplitudes of the input command signals Vuin, Vvin, Vwin by means of the difference between the current detection signal Vcs and the drive command signal Vec, and pulse-width-modulates the switching control signals UPWM, VPWM, WPWM to control the drive current Im that flows in the three-phase coils 71, 72, 73.
A gain of the conventional-motor driving apparatus is described with reference to a block diagram of FIG. 11.
The rotational position detection signals H1, H2, H3 having a voltage amplitude value Vh outputted from the rotational position detector 11 are amplified by the hall amplifier 11b having a voltage amplification coefficient Ah, and signals having a voltage amplitude value of (Ah×Vh) are inputted into the multiplication circuit 2a. 
When a transfer coefficient Ae of the error amplifier circuit 6a is defined as “Ir/(Vec−Vcs)”, the error amplifier current signal Ir expressed by the following expression (1) is inputted into the multiplication circuit 2a. Ir=Ae×(Vec−Vcs)  (1)
The multiplication circuit 2a having a transfer coefficient Am multiplies the signal (Ah×Vh) by the error amplifier current signal Ir to output the input command signals Vuin, Vvin, Vwin of a voltage amplitude value Vd. The voltage amplitude value Vd of the input command signals Vuin, Vvin, Vwin is expressed by the following expression (2).Vd=Am×Ah×Vh×Ir  (2)
A transfer coefficient Gm of the drive circuit 3 with respect to the voltage amplitude value Vd shows a transfer characteristic of the drive current Im with respect to the voltage amplitude value Vd of the input command signal Vuin Vvin, Vwin in a case where prescribed values are given as an amplitude value and a frequency of the carrier wave Vtri. Therefore, the drive current Im is expressed by the following expression (3).Im=Gm×Vd  (3)
The current detection signal Vcs outputted from the current detection circuit 4 is expressed by the following expression (4) using the drive current Im outputted from the drive circuit 3 and a resistance value Rcs of the detection resistor in the current detection circuit 4. In addition, in a case where the detection resistor is not used in the current detection circuit 4, the coefficient Rcs shows a coefficient which converts current to voltage.Vcs=Im×Rcs  (4)
From the expressions (1), (2), (3), the drive current Im is expressed by the following expression (5).Im=Gm×Am×Ah×Vh×Ae×(Vec−Vcs)  (5)
From the expressions (4), (5), the gain (defined as Im/Vec) of the motor driving apparatus is expressed by the following expression (6).Im/Vec=1/{1/(Gm×Am×Ae×Ah×Vh)+Rcs}  (6)
In the above expression (6) for the gain of the drive apparatus, when the first term of the denominator on the right-hand side, 1/(Gm×Am×Ae×Ah×Vh), is very small as compared with Rcs, the first term of the denominator can be ignored. Hence, the gain of the drive apparatus is expressed by the following expression. In this case, the circuit constants of Gm, Am and the like exert no effect.Im/Vec≅1/Rcs  (7)
However, in the case of driving the motor 7 with the small drive current Im, for example in the case of performing drive control while keeping the number of rotation of the motor 7 very small, the voltage amplitude value Vd of the input command signals Vuin, Vvin, Vwin becomes very small. When the voltage amplitude value Vd is small, a value of the transfer coefficient Gm of the drive circuit 3 becomes small, and as a result, the first term of the denominator, 1/(Gm×Am×Ae×Ah×Vh), in the expression (6) becomes large. The value of the first term depends upon the circuit constants of Gm, Am, Ae, Ah and Vh, and since these circuit constants vary, variations in the gain of the motor driving apparatus become large.
As thus described, as for the conventional motor driving apparatus that performs PWM-driving, when the voltage amplitude value Vd of the input command signals Vuin, Vvin, Vwin is large, that is, when the drive current Im is large, the variations in circuit constants of the motor driving apparatus have a small effect as shown in the expression (6), and therefore, the individual motor driving apparatuses can drive a motor with similar characteristics. However, when the voltage amplitude value Vd is small, that is, when the drive current Im is small, the variations in circuit constants of the motor driving apparatus have a large effect to cause variations in the gain to depend upon the individual motor driving apparatuses. There has thus been a problem in that the conventional motor driving apparatus cannot drive a motor with similar characteristics to cause variations in number of rotation of the motor to depend upon the individual motor driving apparatuses.